Study on the deterioration characteristics of aeolian sand concrete under the coupling effect of multiple factors in harsh environments

This study takes the aeolian sand concrete as a research object, uses the relative dynamic elastic modulus to study its macro characteristics, and combines nuclear magnetic resonance、scanning electron microscope to study its pore characteristics and micro morphology under the action of prestress, freeze-thaw and salt intrusion. The results show that with the increase of the amount of aeolian sand, the dynamic elastic modulus of aeolian sand concrete shows a pattern of first decreasing, then increasing, and then decreasing; when no prestress is applied, the porosity of aeolian sand concrete first increases, then decreases, and then continues to increase. Among them, the porosity of aeolian sand concrete with a 40% content of aeolian sand decreases by 0.06% compared to that with a 0% content of aeolian sand, and decreases by 0.003% compared to that with a 60% content of aeolian sand; with the increase of prestress, the porosity of aeolian sand concrete with the same amount of aeolian sand increases gradually with the increase of damage degree. The porosity of concrete with 40% aeolian sand content increases by 0.33% when the damage degree is 0.0 compared to 0.3, with a 6.31% increase in the number of multi damage pores; under the coupling effect of multiple factors, when the amount of aeolian sand is 40%, the damage degree of the four groups of aeolian sand concrete before and after the coupling effect is 0.0, 0.1, 0.2, and 0.3, respectively, increases by 25.8%, 32.2%, 73.8%, and 85.8%, respectively; under the coupling effect of multiple factors, the content of aeolian sand is 60%, the damage degree is 0.2 and 0.3 groups, and the content of aeolian sand is 20%, the damage degree is 0.3 groups, which does not meet the standard requirements; under the coupling action of stress, freeze-thaw, salt intrusion and the amount of aeolian sand, the filling effect of aeolian sand on the internal pores of aeolian sand concrete decreases first, then increases, and then decreases with the increase of the amount of aeolian sand. The filling effect is further weakened after the action of stress. After the superposition of freeze-thaw and salt intrusion, the coupling effect of water and salt solution in frost heaving medium makes the variation law and range of physical and chemical characteristics of aeolian sand concrete show a great difference.


Introduction
The mechanics, frost resistance and other corrosion resistance characteristics of concrete under prestress are quite different from those under conventional service environment.Domestic and foreign scholars have also conducted a large number of experimental and theoretical studies on the deterioration characteristics and mechanism of concrete after stress damage, such as: Zhou Zhijun et al [1].Studied the carbonation resistance of concrete under the action of bending stress and nitrate erosion, and found that with the increase of nitric acid concentration and stress level, the carbonation depth increased, the content of carbonated substances decreased, and nitric acid consumed hydration products, weakened the filling of pores by CaCO 3 , formed microcracks, and increased the porosity, while bending stress promoted the development of microcracks to form tensile cracks, and formed microcracks in coordination with carbonation; Liu Yan et al [2].Studied the carbonation performance of recycled concrete under the action of axial stress, and pointed out that the axial compressive stress suppressed the carbonation damage of recycled concrete, and the axial tensile stress aggravated the carbonation damage of recycled concrete.Under the action of axial compressive stress carbonation, the internal structure of recycled concrete is relatively dense, and under the action of axial tensile stress carbonation, through cracks are formed in recycled concrete; Duan shaozhen [3] and others pointed out in their research on the mechanical properties of concrete-filled steel tubular short columns under axial compression under the action of initial construction stress that the simultaneous existence of initial construction stress of outer steel tube and section steel will delay the interaction time between concrete and steel tube and section steel, weaken the hooping effect of steel tube on concrete and the local constraint of concrete on section steel, make the composite members enter the elastoplastic stage in advance and expand the scope of this stage, the ultimate bearing capacity of composite columns decreases with the increase of initial stress; Tang Guanbao et al [4].Studied the influence of stress on concrete air permeability coefficient and CO2 diffusion coefficient, and established a prediction model of concrete carbonation depth under stress based on air permeability coefficient CDEP GPL model; Wang Jiabin et al [5].Studied the chloride ion diffusion performance in the tensile and compressive areas of shotcrete under the action of bending stress, and pointed out that the stress changes the pore structure distribution of shotcrete and the number and depth of microcracks, which has an impact on the chloride ion diffusion rate.On the side of the tensile area, the chloride ion content increases with the increase of the bending stress ratio; on the side of compression zone, the chloride ion content decreases with the increase of bending stress ratio; Zeng Yitao [6] and others studied the hydraulic fracturing problem when the dam heel has initial cracks due to construction stress based on the combination of different bending moments and water pressure values, and pointed out that a small load increment will break the steady state and promote the unstable expansion of cracks; there is a superposition effect between splitting water pressure and tensile stress.If the maximum action position is the same, the stress concentration at the crack tip is obvious, which is easy to cause hydraulic splitting damage.If the action positions are different, the strain distribution of the tensile section is more uniform, which greatly gives play to the splitting tensile resistance of the concrete tensile section and weakens the hydraulic splitting effect; Tran T.T et al [7].Studied the relationship between water permeability and chloride diffusion coefficient of concrete under compressive stress, proposed a fine-scale hydrodynamic lattice model to simulate the fluid flow and chloride entry in concrete under stress, used a softening damage model to describe the behavior of cement matrix and ITZ, and assumed that the aggregate was elastic, according to the experimental results of water permeability and chloride ion diffusion test, the hydraulic mechanical parameters of concrete members are calibrated, and the relationship between water permeability and diffusion coefficient is obtained; Liu Jin et al [8].Studied the multiscale analysis theory of concrete diffusion coefficient under mechanical stress, proposed a multi-scale analysis model, predicted the chloride ion diffusion rate of concrete under mechanical stress, and evaluated the quantitative relationship between the apparent diffusion coefficient of concrete and mechanical stress (here is volume strain), as well as the current porosity of cement paste and ITZ; the research on the characteristics of ultrasonic wave velocity of concrete under the action of Ling Zhang [9] and other stresses points out that under the same loading conditions, the ultrasonic velocity has an increasing trend with the increase of age for C10 and C20 concrete samples, but for C30, C35, C40 and C45 concrete samples, the overall growth trend of velocity is not obvious; Guo Jun liu et al [10].Studied the failure criterion of concrete under multiaxial stress, analyzed its limitations, and proposed the research direction of concrete strength failure criterion; Yu Chuan Jiang et al [11].Tested and evaluated the chloride permeability of concrete under compressive stress, pointing out that the electric flux of concrete under compressive stress is affected by the stress level.With the increase of stress level, the electric flux of concrete first decreases and then increases; the electric flux of concrete with different strength grades has different sensitivity to compressive stress.The lower the strength grade of concrete, the higher the sensitivity to compressive stress; the electric flux of concrete after compressive stress is related to crack recovery, and only the stress level of more than 60% has a significant impact on the electric flux of concrete; the load should be considered in the evaluation of chloride permeability of concrete structures; Jian Hui Yang et al [12].Studied the constitutive relationship of concrete under plane stress based on the generalized octahedral theory, and pointed out that the plane constitutive relationship of concrete can be understood through the plane problem of stress-strain space transformation.According to the symmetry of an orthogonal octahedral, one of the arbitrary inclined planes is parallel to one of the three rectangular coordinate axes.
To sum up, scholars at home and abroad have studied the deterioration process of concrete under uniaxial, biaxial and bending stresses, and made clear its superimposed influence on the coupled erosion effects such as salt intrusion and carbonation, and pointed out that when stress is coupled with other erosion effects, concrete deterioration presents phased changes, and the promotion or suppression of concrete deterioration also depends on the superimposed effect, based on damage theory-Fick's law Octahedral theory established the deterioration model under the coupling effect of stress and erosion.However, the existing research often focuses on the traditional concrete materials, and the coupling factors are also common in the two factors coupling.The damage mechanism research is also dominated by the deterioration mechanism under the action of a single factor.There are still significant deficiencies in the research and development of green new building materials, as well as in the study of the deterioration mechanism of concrete materials under the coupling of multiple factors, which is in line with the harsh environment in the western region.
In view of this, this study uses aeolian sand widely distributed in Western China as raw material to replace river sand to prepare a new green building material, namely aeolian sand concrete, and discusses its degradation process under multiple factors in the harsh environment of the western region (stress, freeze-thaw, and salt intrusion), so as to clarify its degradation characteristics and mechanism, which is not only conducive to the development of aeolian sand resources, the economic development of aeolian sand, and the research and development of new building materials, it is also conducive to protecting river sand resources, promoting social development and environmental protection, which is of great social, economic and practical significance.

Introduction to raw materials
The fine aggregate used in the study is from the sand field around Hohhot, with a fineness modulus of 2.91 and a particle size range of 0.075~4.75mm;aeolian sand is taken from the Kubuqi Desert in Inner Mongolia, China, with a fineness modulus of 0.72.The coarse aggregate used in the test is egg gravel, with an apparent density of 2669kg/m 3 , a bulk density of 1650kg/m 3 , and a particle size range of 4.75~20.0mm;the mixing water is ordinary tap water; the water reducing agent adopts polycarboxylic acid sc-40 high-efficiency water reducing agent of Inner Mongolia Rongshengda new materials Co., Ltd., with a water reducing rate of 26%; the air entraining agent is sj-3 high-efficiency air entraining agent, grade II fly ash of Inner Mongolia Jinqiao power plant and Jidong P•O42.5 cement.Some physical and chemical property indexes are shown in Tables 1-4 below.

Mix proportion design of aeolian sand concrete
In accordance with the relevant provisions of concrete mix proportion design in the code for hydraulic concrete construction (SL677-2014), code for mix proportion design of ordinary concrete JGJ55-2011) and ACI method of proportion, and in order to ensure that the aeolian sand concrete can obtain high slump and fluidity for engineering practice, equivalent replacement shall be carried out according to the aeolian sand content of 0%, 20%, 40%, 60%, 80% and 100%, the strength grade of the benchmark aeolian sand concrete is generally C25, and its mix is shown in Table 5.

Introduction to coupling conditions
According to the standard for test methods of long-term performance and durability of ordinary concrete > [13] (GBT50082-2009), aeolian sand concrete with different amounts of aeolian sand is selected to design the coupling conditions of stress, freeze-thaw and salt intrusion, as follows: ① Prestressing: select three groups of 100 mm×100 mm×400mm aeolian sand concrete prism specimens with different amounts of aeolian sand (specifically increase or decrease the number of groups according to the mechanical property test results), measure the initial relative dynamic elastic modulus, and define the damage degree at this time as 0.0.Apply axial loads respectively by universal testing machine.When the relative dynamic elastic modulus decreases by 5%, stop loading, and define the damage at this time as 0.1, and so on, the relative dynamic elastic modulus of aeolian sand concrete with damage degree of 0.2 and 0.3 is 90% and 85% of the initial value respectively.
② Apply freeze-thaw and salt intrusion: select concrete specimens of aeolian sand with different amounts of aeolian sand and different degrees of damage.After 24 days of standard curing, immerse them in (full immersion method) the freeze-thaw medium (5.0%NaCl solution) with a temperature of 15 ~20˚C.After 4 days, wipe the surface moisture of the soaked specimens, and further determine the relative dynamic elastic modulus at this time.
Then, use the "quick freezing method((Frozen at -20~-10 for 2h, melted at 15-20 for 2h))" to carry out the freeze-thaw and salt intrusion coupling test of aeolian sand concrete.
③ Test and analysis: under the coupling effect of stress, freeze-thaw and salt intrusion, the relative dynamic elastic modulus of aeolian sand concrete samples with different amounts of aeolian sand is measured with 25 freeze-thaw cycles as a cycle.When the relative dynamic elastic modulus drops to 60% of the initial value, the test is terminated, and then the micro pore structure after prestressing and the coupling effect of stress, freeze-thaw and salt intrusion is measured by NMR, Sigma500 field emission scanning electron microscope was used to measure the micro morphological characteristics before and after the coupling condition.
3 Test results and analysis

Mechanics and pore characteristics of aeolian sand concrete before and after stress action
The compressive strength and splitting tensile strength of aeolian sand concrete are measured according to the standard for test methods of mechanical properties of ordinary concrete GB/ T50081-2002, as shown in Figs 1 and 2 below.According to Figs 1 and 2, when the content of aeolian sand is 0%, 20%, 40%, 60%, 80% and 100%, the 7d unconfined compressive strength and 28 d splitting strength first increase and then decrease with the increase of the content of aeolian sand, and the 28 d unconfined compressive strength gradually decreases with the increase of the content of aeolian sand.When the content of aeolian sand is 80%, the compressive strength is close to the standard value, and if the content of aeolian sand continues to increase, the compressive strength has been slightly lower than the standard requirements.In view of this, according to the analysis results of mechanical properties, the aeolian sand concrete with the content of 0%, 20%, 40% and 60% (groups I, II, III and IV) is selected for the subsequent stress, freeze-thaw and salt invasion coupling test, and its elastic modulus, porosity and other relevant physical and chemical indicators are measured, as shown in Fig 3 and Table 6 below.
It can be seen from Fig 3 that with the increase of the amount of aeolian sand, the relative dynamic elastic modulus of aeolian sand concrete decreases first, then increases, and then decreases, but all meet the requirements of C25 concrete standard.According to the results of pore characteristics in Table 6, when prestress is not applied (the damage degree is 0.0 group), the porosity of aeolian sand concrete first increases, then decreases, and then continues to increase.The porosity of aeolian sand concrete with 40% of aeolian sand content is 0.06% lower than that with 0% of aeolian sand content, and 0.003% lower than that with 60% of aeolian sand content, and the pore changes are mainly concentrated in the harmless pores of 20-50nm [14], the content of aeolian sand with 40% is the highest, reaching 27.05%, and the number of harmful holes above 200nm is the least, only 44.79%.This is because the fineness modulus of aeolian sand is 0.72, which is far lower than 2.91 of ordinary sand.When the amount of aeolian sand is small, finer aeolian sand can fill the internal pores of aeolian sand concrete, reduce the porosity, increase the number of harmless pores, and reduce the number of harmful pores.However, with the increase of the amount of aeolian sand, the filling effect of aeolian sand decreases, and finer aeolian sand reduces the workability of aeolian sand concrete and increases the water consumption of standard consistency, at this time, the density of the aeolian sand concrete formed by mixing decreases, and the porosity increases, and most of them are harmful pores above 200nm.At the same time, it can be seen from Table 6 that when prestress is applied, with the increase of prestressing, the porosity of aeolian sand concrete with the same amount of aeolian sand presents a gradual increasing trend.When the damage degree is 0.0, the porosity of aeolian sand concrete with 40% of aeolian sand is 1.598%, while when the damage degree is 0.3, the porosity increases to 1.928%, with an increase of 0.33%, in which the number of harmful holes increases by 6.31%, and the irreducible fluid saturation decreases by 2.371%.At the same time, according to the indoor macro test, the compressive strength of aeolian sand concrete with 40% content of aeolian sand also decreases after prestress, of which the compressive strength is 25.2MPa when the damage degree is 0.3, which is 9.6% lower than the initial value of 27.9MPa.This is because under the action of prestress, the small harmless and less harmful holes in the aeolian sand concrete are destroyed with the increase of stress, and then transformed into harmful and more harmful holes.At this time, the NMR test results show that the irreducible fluid saturation also gradually decreases, which further proves that the proportion of small holes in the aeolian sand concrete decreases; the mechanical properties of aeolian sand concrete decrease with the increase of porosity.

Test results and analysis of aeolian sand concrete under the coupling effect of stress, freeze-thaw, salt intrusion and aeolian sand content
On the basis of the study on the mechanical properties of aeolian sand concrete and the pore characteristics after prestress, the concrete with damage degrees of 0.0, 0.1, 0.2 and 0.3 and the content of aeolian sand of 0%, 20%, 40% and 60% was first soaked in 5.0% sodium chloride solution for 4 days, and then put into the freeze-thaw test box with 5.0% sodium chloride solution as the freeze-thaw medium for stress, freeze-thaw and salt intrusion coupling tests, taking 25 freeze-thaw cycles as a cycle, measuring the relative dynamic elastic modulus of aeolian sand concrete.When the relative dynamic elastic modulus drops below 60% of the initial value or the freeze-thaw cycle is 200 times, the test is terminated, and samples are taken for NMR,  7.
It can be seen from Figs 4 and 5 that the compactness of aeolian sand concrete under different damage degrees and the same amount of aeolian sand gradually decreases.After 200 times of stress, freeze-thaw and salt intrusion coupling, the relative dynamic elastic modulus of aeolian sand concrete with the same damage degree and different amounts of aeolian sand shows a downward trend as a whole, further indicating that the internal compactness of aeolian sand concrete under coupling conditions has decreased, the relative dynamic elastic modulus of aeolian sand concrete with 60% aeolian sand content decreased to 58% and 54% of the initial value respectively when the damage degree was 0.2 and 0.3, and the damage degree was 0.3.The relative dynamic elastic modulus of aeolian sand concrete with 20% aeolian sand content also decreased to 59% of the initial value, which was lower than the standard requirements, while other groups were higher than the standard value.At the same time, after the coupling working condition, the relative dynamic elastic modulus of aeolian sand concrete under the same damage degree shows a change law of first decreasing, then increasing, and then decreasing with the increase of the content of aeolian sand.For different damage degrees, with the increase of the damage degree, the relative dynamic elastic modulus after the coupling working condition decreases significantly, among which, when the content of aeolian sand is 40%,  before and after the coupling, the relative dynamic elastic modulus of aeolian sand concrete with damage degree of 0.0, 0.1, 0.2 and 0.3 decreased by 23.8%, 24.4%, 26.0% and 38.8% respectively, and the relative dynamic elastic modulus of the damage degree of 0.0 group was 5.8%, 17.4% and 31.7% higher than that of 0.1, 0.2 and 0.3 groups respectively.It can be seen from Tables 6 and 7 that after 200 times of stress, freeze-thaw and salt invasion, the porosity of aeolian sand concrete increases and the irreducible fluid saturation decreases compared with the initial value.After the coupling effect, under the same damage degree, the porosity of aeolian sand concrete as a whole shows a trend of first increasing, then decreasing, and then increasing with the increase of aeolian sand content, and the main body of its pore size distribution shows that the proportion of harmless and less harmful pores decreases, and the proportion of harmful and more harmful pores increases, while the bound fluid saturation first decreases, then increases, and then decreases with the increase of aeolian sand content.Under different damage degrees, with the increase of damage degree, the porosity of aeolian sand concrete shows a significant increasing trend.When the content of aeolian sand is 40%, the concrete porosity of the four groups of aeolian sand with damage degrees of 0.0, 0.1, 0.2 and 0.3 before and after coupling increases by 25.8%, 32.2%, 73.8% and 85.8% respectively, while the irreducible fluid saturation decreases by 5.3%, 25.8%, 27.7% and 41.2% respectively; after the coupling effect, the porosity of 0.0 group is 11.8%, 49.8% and 77.8% lower than that of 0.1, 0.2 and 0.3 groups respectively, and the irreducible fluid saturation of 0.0 group is 20.3%, 22.4% and 40.8% higher than that of 0.1, 0.2 and 0.3 groups respectively.This is due to the unique physical and chemical characteristics of aeolian sand concrete different from ordinary concrete under the coupling action of stress, freeze-thaw, salt intrusion and the change of aeolian sand content.When aeolian sand replaces river sand, if the replacement proportion is less, aeolian sand, as a super fine sand, requires more water in the concrete slab and process, and because the amount of aeolian sand is less, its effect on filling the pores in the concrete is poor, resulting in an increase in the overall porosity.Therefore, when the amount of aeolian sand is 20%, the porosity of aeolian sand concrete increases, and the relative dynamic elastic modulus and irreducible fluid saturation decrease within the same damage degree; with the increase of the replacement proportion of aeolian sand, the filling effect of aeolian sand on the internal pores of concrete increases, so when the content of aeolian sand is 40%, the porosity of aeolian sand concrete in the same damage degree decreases, and the relative dynamic elastic modulus and irreducible fluid saturation increase; at the same time, when the amount of aeolian sand is large, the filling effect of aeolian sand on the pores in the concrete has reached saturation, and the excess aeolian sand needs more water during the preparation of concrete, resulting in the formation of aeolian sand particles including incomplete mixing in the concrete.Therefore, when the amount of aeolian sand is 60%, the internal porosity of aeolian sand concrete increases significantly, and the relative dynamic elastic modulus and bound fluid saturation decrease significantly.
When the stress is superimposed, it is only the mechanical damage to the original concrete, which reduces the physical and chemical properties of aeolian sand concrete to a certain extent, but does not change the change rules of the physical and chemical properties such as the relative dynamic elastic modulus and porosity of aeolian sand concrete caused by the change of the content of aeolian sand; when the freeze-thaw and salt intrusion are superimposed, the change law and range of physical and chemical properties of aeolian sand concrete show great differences.This is because under the action of freeze-thaw and salt intrusion, the freeze-thaw medium gradually erodes into the interior of aeolian sand concrete, and the water in the freeze-thaw medium freezes and melts under the action of freeze-thaw, while the volume of water expands when it freezes, destroying the internal pores of aeolian sand concrete, make it transform from harmless and less harmful pores to more harmful and harmful pores with larger pores.At the same time, the salt in the freeze-thaw medium enters the interior of aeolian sand concrete with the pores generated by water frost heaving and melting under the action of capillary, infiltration and diffusion [15][16][17][18][19], and produces Friedel salt and other crystallization products, which accelerates the generation and destruction of interface cracks between cement paste and aggregate, further expands the pores generated by frost heaving, and starts again and again, the concrete compactness of aeolian sand decreases, the relative dynamic elastic modulus decreases, the porosity increases, and the irreducible fluid saturation increases.

Conclusion
1.With the increase of the amount of aeolian sand, the relative dynamic elastic modulus of aeolian sand concrete decreases first, then increases, and then decreases.The porosity increases first, then decreases, and then continues to increase.The porosity of aeolian sand concrete with 40% of aeolian sand is 0.06% lower than that with 0% of aeolian sand, and 0.03% lower than that with 60% of aeolian sand, and the pore changes are mainly concentrated between harmless pores of 20-50nm, up to 27.05%.
2. With the increase of prestress, the porosity of aeolian sand concrete with the same amount of aeolian sand presents a gradual increase trend.When the damage degree of aeolian sand concrete with 40% of aeolian sand is 0.0, the porosity is 1.598%, while when the damage degree is 0.3, the porosity increases to 1.928%, with an increase of 0.33%, in which the number of more harmful pores increases by 6.31%, and the irreducible fluid saturation decreases by 2.371%.This indicates that pre-stressing causes the transformation of less harmful pores with smaller internal pore sizes into harmful pores with larger pore sizes, resulting in larger pores and a decrease in compactness in aeolian sand concrete.
3. Under the action of stress, freeze-thaw and salt intrusion, when the content of aeolian sand is 60%, the damage degree is 0.2 and 0.3, it decreases to 58%, 54% .And when the content of aeolian sand is 20%, the damage degree is 0.3, it decreases to 59% of the initial value, and the other groups meet the standard requirements; when the content of aeolian sand is 40%, the damage degree is 0.0, 0.1, 0.2 and 0.3.The relative dynamic elastic modulus of aeolian sand concrete decreases by 23.8%, 24.4%, 26.0% and 38.8% respectively, the porosity increases by 25.8%, 32.2%, 73.8% and 85.8% respectively, and the irreducible fluid saturation decreases by 5.3%, 25.8%, 27.7% and 41.2% respectively, meet the standard requirements.

Fig 1 .
Fig 1. Relationship curves of 7 d and 28 d of the aeolian sand concrete, the unconfined compressive strength and the volume of the aeolian sand.https://doi.org/10.1371/journal.pone.0289847.g001

Fig 5 .
Fig 5. Micromorphology of aeolian sand concrete with different damage degrees after the coupling action of stress, freeze-thaw and salt intrusion.a.The damage degree after stress, freeze-thaw and salt intrusion is 0.0, and the content of aeolian sand is 40%.The results of electron microscope test of aeolian sand concrete.b.The damage degree after stress, freeze-thaw and salt intrusion is 0.1, and the content of aeolian sand is 40%.The results of electron microscope test of aeolian sand concrete.c.The damage degree after stress, freeze-thaw and salt intrusion is 0.2, and the content of aeolian sand is 40%.The results of electron microscope test of aeolian sand concrete.d.The damage degree after stress, freeze-thaw and salt intrusion is 0.4, and the content of aeolian sand is 40%.The results of electron microscope test of aeolian sand concrete.https://doi.org/10.1371/journal.pone.0289847.g005

Table 4 . The physical and dynamic parameters of experiment fly ash.
https://doi.org/10.1371/journal.pone.0289847.t004field emission scanning electron microscope.The test results are shown in Figs 4 and 5 and Table